The present invention relates to a reduction-distribution unit particularly suitable for industrial machines.
As is known, gearshift units are frequently used in industrial machines, for example earth-moving machines; those units are formed by a variable-displacement hydrostatic pump connected to the thermal engine and by a variable-displacement hydrostatic motor which is connected to the wheels.
This known solution has the severe disadvantage of having a limit in the obtainable conversion ratio, which is approximately equal to three times, by adjusting the displacement of the pump, and to another three times by adjusting the displacement of the motor, for a total of approximately nine times.
This aspect limits the maximum speed of these vehicles to approximately 18 km/h with an operating speed of approximately 2 km/h under maximum traction stress.
Power-shift units with two-three speeds have been studied in order to try and obviate this limit, and ensure a working speed comprised between 2 and 10 km/h, a possible intermediate speed comprised between approximately 4 and 20 km/h and a travel speed which is comprised between 7.6 and 38 km/h.
A problem shown by power-shift units mounted downstream of the hydrostatic motor is constituted by the inertia in the variation of the displacement of the pump or of the hydrostatic motor during power-shifting, which is a necessary condition in order to vary the rotation rate due to the non-compressibility of the fluid. It is therefore necessary to intervene synchronously both on the power-shift unit and on the hydrostatic unit, which must necessarily be equipped with an electronic adjustment; quality in the gear shifting step is generally unsatisfactory.
As an alternative, two hydrostatic motors with conventional adding gear systems have also been used; said motors exploit the possibility of making one of the two motors run idle at high rpm, thus increasing the conversion ratio, since at low speeds both motors operate and add their torques proportionately to their displacement, distributing the flow-rate of the pump.
However, the motor which is driven idle at high rpm absorbs considerable power, so that other solutions have been sought to improve the system by disconnecting the inoperative motor by means of a clutch and by locking it with a brake.
All the above described solutions have however generally turned out to be complicated from a constructive point of view and to have a relatively low efficiency.